He published a book about cold prevention using megadoses of vitamin C, or up to 18,000 mg daily. For comparison, the RDA is 75 mg for women and 90 mg for men.

At that time, no reliable studies had proved this to be true.

But in the following few decades, multiple randomized controlled studies examined whether the vitamin had any effect on the common cold.

The results have been fairly disappointing.

An analysis of 29 studies including 11,306 participants concluded that supplementing with 200 mg or more of vitamin C did not reduce the risk of catching a cold (1Trusted Source).

However, regular vitamin C supplements had several benefits, including:

Reduced cold severity: They reduced the symptoms of a cold, making it less severe.

Reduced cold duration: Supplements decreased recovery time by 8% in adults and 14% in children, on average.

A supplemental dose of 1–2 grams was enough to shorten the duration of a cold by 18% in children, on average (Trusted Source).

Other studies in adults have found 6–8 grams per day to be effective (2Trusted Source).

Vitamin C appears to have even stronger effects in people who are under intense physical stress. In marathon runners and skiers, vitamin C alantost halved the duration of the common cold (Trusted Source).